It may take a new light-rail bridge with cycling access to truly ease bike congestion on Portland's bridges.

After reading this week's column on bike traffic jams on the Hawthorne Bridge, Patty Collins of Ridgefield, Wash., called with "an obvious solution" to keep cyclists from riding off the elevated, 10-foot-wide sidewalk into auto traffic.

"Why don't they just install a railing or some kind of fencing along the edge?" she asked. "It seems to make sense and it wouldn't be that expensive."

Actually, bike commuters and pedestrians sharing the increasingly crowded paths over the Willamette River span have been e-mailing and calling Multnomah County with that suggestion for a few years.

The reality, said county transportation spokesman Mike Pullen, is that there is no way to effectively add a barrier without either making the traffic lane too narrow for buses or taking about foot of space from the crowded sidewalk.

As it is, buses can just barely move across the bridge on the grated lanes adjacent to the sidewalks, he said.

"It would take a foot away in either direction," Pullen said. "We would be squeezing things even more."

Of course, the May 6 crash of cyclist Erica Rothman underscores the danger. During the evening commute, another cyclist attempted to pass Rothman, a 24-year-old social worker, and bumped her handlebars. She lost control, rode off the sidewalk and crashed into the traffic lane - in front of an oncoming car.

Doctors stitched and stapled Rothman's gashes. And she has since returned to commuting by bike, with the help of about $300 raised by BikePortland.org to help her get her Trek 390 fixed and pay her medical bills.

But the county says they can't do much more to help when it comes to making the Hawthorne safer for the 7,400 cyclists who cross it daily.

The 98-year-old bridge wasn't built for that kind of two-wheeled congestion. Bicycles now make up 20 percent of the bridge's traffic, largely because it links to Southeast Portland's extensive network of bikeways as well as the Springwater Corridor Trail.

Of course, it's not just the Hawthorne. Since 2003, daily bike traffic on the Steel Bridge has jumped from 1,900 to 3,000 riders. The Burnside's bike count has more than double to 2,000 bikes a day during the same period.

Last week, I was forced to stop midspan on the Broadway for a bridge lift while I was riding my bike to work. Ten minutes later, as the platform drifted back into place and the bridge reopened, I counted 69 bike riders waiting behind me. That was just 10 minutes in the middle of the day.

Aside from Rothman's alarming crash and injuries, the bike jams on the bridges are "a good problem to have," said Roger Geller, Portland's bicycle coordinator in the Bureau of Transportation. "It's indicative of the volumes of people using bicycles for transportation."

Indeed, in his blog post linking to my column, "Traffic" author Tom Vanderbilt remarked that we are becoming "Copenhagen on the Willamette." Meanwhile, the Portlandize blog lamented about how Portland's growing bike congestion has led to a breakdown of etiquette in bike parking areas.

The county has considered designating the sidewalk on one side of the Hawthorne Bridge for bikes only and the other sidewalk for foot traffic. But given how access to the Eastbank Esplanade works and the likely problems managing such traffic, the county sees such a solution as unworkable, Mullen said.

In the near future, Mullen said the county is counting on bike improvements to the Morrison and the Sellwood Bridges to help ease the Hawthorne's congestion.

The report from TriMet anticipates that the daily volume of bicycles on the light-rail bridge will be as much as 5,900 when it opens in 2015 and up to 15,200 by 2030. Rob Barnard of TriMet said the actual number in the next 20 years might be closer to 8,300 bikes a day, but the agency wants to plan for the maximum volume to avoid future problems like those on the Hawthorne.

Of course, an anonymous caller who said she commutes by walking across the bridges every day said cyclists could help themselves by slowing down and using their bike bells. Or simply shouting out an alert.

"Most of them don't call out, 'Behind you!' Or 'Hello!' Or 'Bike coming!" the reader said in her voicemail. "I wish they would. It would avoid a lot of close calls that I see between bikes and pedestrians."

Rothman agrees. The bicyclist who bumped into her as she was riding on the Hawthorne didn't slow or give her a warning as he approached from behind, she said. "We have at least one common interest - bicycling," she said. "It should be enough for a little mutual respect."